The present invention relates to a sensor and method for measuring changes in environmental conditions. In particular, the present invention relates to a sensor and method using an optical interferometer for an accurate determination of small changes in pressure, temperature or the like.
In the aerospace industry sensors are used to make critical measurements for flight control, engine control and cockpit display systems. These sensors include pitot-static probes, air temperature sensors, engine temperature sensors, pressure sensors, angle-of-attack sensors and stall warning sensors. Interferometers have been used in some sensors for measuring the selected parameters. An example of an interferometer is a Fabry-Perot interferometer, which has two partially reflective surfaces that are separated from one another. The environmental condition or parameter to be measured is operably coupled to one or both of the reflective surfaces such that a change in the parameter changes the distance between the surfaces or the optical properties of the media between the surfaces. The interferometer measures the distance between the reflective surfaces based on interference between light reflecting from the surfaces. A variety of different interferometric sensors make use of this principle.
The sensor and method of the present invention measure a change in an applied environmental condition relative to a reference environmental condition. In one aspect of the present invention, the method includes: operating an interferometer under the applied environmental condition to generate an interference spectrum; measuring intensities of the interference spectrum at first and second wavelengths, wherein the first and second wavelengths correspond to first and second reference intensities that are separated from one another by an odd number of extrema in the interference spectrum when the interferometer is operated under the reference condition; and measuring the change in the applied environmental condition based on the measured intensities at the first and second wavelengths and the first and second reference intensities.
In another aspect of the present invention, the sensor includes optical elements defining an optical path operably coupled to the environmental condition upon exposure of the sensor to the environmental condition. A polychromatic light source is optically coupled to first and second reflective surfaces. A spectral element is optically coupled to the first and second reflective surfaces and spatially disperses light received from the first and second reflective surfaces as a function of wavelength. First and second light sensitive elements are optically coupled to the spectral element and are positioned to receive light from the spectral element at selected first and second respective wavelength ranges. The first and second light sensitive elements have outputs which are representative of light intensity within the first and second wavelength ranges. A measurement apparatus measures the change in the environmental condition based on the outputs of the first and second light sensitive elements and based on first and second reference values. The first and second reference values represent light intensities within the first and second wavelength ranges that are separated from one another by an odd number of extrema in intensity as a function of wavelength of the light spatially dispersed by the spectral element, when the sensor is exposed to a reference environmental condition.
Another aspect of the present invention is directed to a method of calibrating a sensor for measuring changes in an environmental condition. The method includes: selecting an expected value of the environmental condition as a reference condition; operating a polychromatic interferometer under the reference condition to obtain an interference pattern in terms of an intensity as a function of wavelength; choosing first and second wavelengths that are separated from one another by an odd number of local extrema in the intensity as a function of wavelength; and storing a representation of the intensity of the interference pattern at the first and second chosen wavelengths